1. Field of Invention
The present invention is generally related to absorber and evaporator heat exchangers in an absorption heat pump. The absorber and evaporator heat exchangers are configured as flat plates that have falling films of either absorbent or refrigerant flowing down their surfaces.
2. Description of the Prior Art
Although electricity is still the primary energy source for our country's air conditioners, chillers and heat pumps, the development of high efficiency absorption systems is leading to a switch from electricity to natural gas in these applications. Since the electric distribution and transmission systems are overburdened in many parts of the country by the summer demand for cooling, and since the natural gas absorption systems use less primary energy in some applications, the switch from electric air conditioners, chillers and heat pumps to systems powered primarily by gas has benefits for the consumer, the utility and the country.
Almost all gas-fired air conditioners, chillers and heat pumps that are now in service are one of two types: engine-driven, vapor-compression systems or absorption systems. Engine-driven vapor-compression systems operate similarly to their electric counterparts with the major difference being that an internal combustion engine, rather than an electric motor, drives the system's compressor.
Absorption systems are fundamentally different than vapor-compression systems. Both systems must raise the pressure of a refrigerant vapor so that it condenses at a temperature that is higher than that of a convenient energy sink (e.g., outdoor air, or water from a cooling tower). However, instead of using a compressor, an absorption system first absorbs the refrigerant vapor into a liquid at low pressure, pumps the liquid up to a high pressure, and then heats the liquid to drive off the refrigerant at the high pressure.
Major obstacles to the wider use of absorption systems are their large size, heavy weight and high cost, which are attributable to the very large heat exchangers that these systems use. It would therefore be a significant advance in the art of absorption systems to reduce the size, weight and cost of absorbers and evaporators--which are the two largest heat exchangers in the absorption system.
Furthermore, absorption systems that use a lithium bromide solution as the absorbent are now practical only in applications where they provide chilled water and reject heat via a cooling tower. This effectively prevents them from being applied in small cooling systems (i.e., cooling capacities under 50 tons), which typically are cooled by outdoor air and deliver cooled air to the building. It would therefore be a significant advance if the absorber of an lithium bromide system was designed so that it could be cooled directly by air and the evaporator was designed so that it could directly cool indoor air.
The most common absorption heat pumps that use solutions of lithium bromide as the absorbent are chillers that reject heat to cooling water (typically water from a cooling tower) and deliver chilled water to the building that is to be cooled. Since chillers are rarely used to cool small buildings, mostly large absorption chillers (cooling capacities of 100 tons or greater) are being manufactured and sold. If a practical absorption heat pump that directly cooled air while rejecting heat to the atmosphere could be developed, lithium-bromide absorption heat pumps could greatly expand their market to include smaller tonnage air conditioners.
An absorption heat pump that directly cools air has been the objective of several R&D efforts. Examples of these are illustrated by Ohuchi, Y., "Development of a Gas-Fired Absorption Heat Pump," pp. 292-303, ASHRAE, Trans., Paper No. 2908, 1983; Biermann, W., "Candidate Chemical Systems for Air Cooled, Solar Powered Absorption Air Conditioner Design, Part III--Lithium salts with Anti-Freeze Additives," DOE Contract No. EG-77-C-03-1587, Carrier Corp., June 1978; and Kurosawa, S., "Development of Air-Cooled Small Sized Gas Absorption Chiller-Heater," IEA Newsletter, Vol.6, No. 4, December 1988.
A problem addressed in the foregoing is the very low heat transfer coefficients on the air-side of the evaporator and absorber, which significantly degrade the system's performance. Up until now, the approach most frequently taken has been to develop new absorption working fluids that have lower equilibrium water vapor pressures, i.e., fluids that can maintain a low evaporator temperature while they themselves operate at a relatively high temperature. These new fluids would permit the absorber to operate at a higher temperature--increasing the driving potential for heat rejection and compensating for the low heat transfer coefficients. Both Biermann (Biermann, W., "Candidate Chemical Systems for Air Cooled, Solar Powered Absorption Air Conditioner Design, Part III--Lithium salts with Anti-Freeze Additives," DOE Contract No. EG-77-C-03-1587, Carrier Corp., June 1978) and Ohuchi (Ohuchi, Y., "Development of a Gas-Fired Absorption Heat Pump," pp. 292-303, ASHRAE Trans., Paper No. 2908, 1983) followed this approach.
Kurosawa (Kurosawa, S., "Development of Air-Cooled Small Sized Gas Absorption Chiller-Heater," IEA Newsletter, Vol.6, No. 4, December 1988) attempted to overcome the performance problems of an air-cooled absorption machine by greatly increasing the surface area of the absorber. This was done by making the absorber from vertical finned tubes that had air flowing over the outside and falling films of lithium bromide solution on the inside. Unfortunately, the Kurosawa design has not produced a manufacturable air-cooled absorption air conditioner.
Thus, a problem associated with common electrically driven air conditioning units is that they consume electricity during summer months of peak demand, and can lead to shortages of electricity and the accompanying brownouts that can occur.
Another problem associated with common electrically driven air conditioning units is that they are extremely expensive to operate, especially in municipalities in which the price of electricity is raised in the summer as a disincentive to air conditioner usage.
More specifically, a problem associated with absorber and evaporator heat exchangers in an absorption heat pump that precede the present invention is that they are prohibitively large for universal application, and hence can only be used in large scale applications.
Yet another problem associated with absorber and evaporator heat exchangers in an absorption heat pump that precede the present invention is that they are extremely heavy, again limiting their applicability in instances other than very large scale applications.
Still a further problem associated with absorber and evaporator heat exchangers in an absorption heat pump that precede the present invention is that they are expensive to build and maintain, which further limits their applicability in instances other than very large scale applications.
For the foregoing reasons, there has been defined a long felt and unsolved need for an absorber and evaporator heat exchangers in an absorption heat pump that is inexpensive to manufacture and can be modified to accommodate a variety of different applications.